Explaining Retrograde Orbits

byPaul GilsteronJanuary 29, 2013

While radial velocity and transit methods seem to get most of the headlines in exoplanet work, there are times when direct imaging can clarify things found by the other techniques. A case in point is the HAT-P-7 planetary system some 1000 light years from Earth in the constellation Cygnus. HAT-P-7b was interesting enough to begin with given its retrograde orbit around the primary (meaning its orbit was opposite to the spin of its star). Learning how a planet can emerge in a retrograde orbit demands learning more about the system at large, which is why scientists from the University of Tokyo began taking high contrast images of the HAT-P-7 system.

It had been Norio Narita (National Astronomical Observatory of Japan) who, in 2008, discovered evidence of HAT-P-7b’s retrograde orbit. Narita’s team has now used adaptive optics at the Subaru Telescope to measure the proper motion of what turns out to be a small companion star now designated HAT-P-7B. The team was also able to confirm a second planet candidate that had been first reported in 2009. The latter, a gas giant dubbed HAT-P-7c, orbits between the orbits of the retrograde planet (HAT-P-7b) and the newfound companion star.

Image: HAT-P-7 and its companion star in images obtained with the Subaru Telescope. IRCS (Infrared Camera and Spectrograph) captured the images in J band (1.25 micron), K band (2.20 micron), and L’ band (3.77 micron) in August 2011, and HiCIAO captured the image in H band (1.63 micron) in July 2012. North is up and east is left. The star in the middle is the central star HAT-P-7, and the one on the east (left) side is the companion star HAT-P-7B, which is separated from HAT-P-7 by more than about 1200 AU. The companion is a star with a low mass only a quarter of that of the Sun. The object on the west (right) side is a very distant, unrelated background star. (Credit: NAOJ)

This Subaru Telescope news release notes the current thinking of Narita’s team on how the retrograde orbit emerged in this system. Key to the puzzle is the Kozai mechanism, first described in the 1960s, which has been found to explain the irregular orbits of everything from irregular planetary moons to trans-Neptunian objects, and has been applied to various exoplanets. The Kozai mechanism says that orbital eccentricity can become orbital inclination, with perturbations leading to the periodic exchange of the two. In other words, what had been a circular but highly-inclined orbit can become an eccentric orbit at a lower inclination.

Ultimately, the effects can be far-reaching as planetary orbits change over time. Can the process be sequential? In the HAT-P-7 system, the researchers are suggesting that the companion star (HAT-P-7B) affected the orbit of the newly confirmed planet HAT-P-7c through the Kozai mechanism, causing orbital eccentricity to be exchanged for inclination. With its orbit now significantly inclined in relation to the central star, Hat-P-7c then affected the inner planet (HAT-P-7b) through the same mechanism, causing its orbit to become retrograde.

The researchers go on to make the case for direct imaging to check for stellar companions that can have a significant effect on planetary orbits. From the paper:

Thus far, the existence of possible faint outer companions around planetary systems has not been checked and is often overlooked, even though the Kozai migration models assume the presence of an outer companion. To further discuss planetary migration using the information of the RM [Rossiter-McLaughlin] eﬀect / spot-crossing events as well as signiﬁcant orbital eccentricities, it is important to incorporate information regarding the possible or known existence of binary companions. This is also because a large fraction of the stars in the universe form binary systems… Thus it would be important to check the presence of faint binary companions by high-contrast direct imaging. In addition, if any outer binary companion is found, it is also necessary to consider the possibility of sequential Kozai migration in the system, since planet-planet scattering, if it occurs, is likely to form the initial condition of such planetary migration.

We have much to learn about retrograde orbits and the rippling effects of the Kozai mechanism are a possibility that will have to be weighed against other observations. Whether the researchers can make this case stick or not, the fact that so many ‘hot Jupiters’ are themselves in highly inclined or even retrograde orbits tells us how important it will be to work these findings into our theories of planet formation and migration. Direct imaging from the SEEDS project (Strategic Exploration of Exoplanets and Disks with Subaru Telescope) should prove useful as we continue to work on direct imaging of exoplanets around hundreds of nearby stars.

Rogue or ophaned planets been captured by a Star is rather difficult to achieve, if they were ejected from their host star they will most likely be travelling very fast, the more likely route was an interaction with one or more other bodies in their parent system, this is what most likely happened to Triton around Neptune which is also retrograde.

Although it may be difficult to capture a rogue planet, according to an article on spaceref.com (Posted Tuesday, April 17, 2012), at least one study indicates that it’s very common. (See the original article I am quoting from at http://www.spaceref.com/news/viewpr.html?pid=36741) “The study, co-authored by Perets and Thijs Kouwenhoven of Peking University, China, appeared in the April 20th 2012 issue of The Astrophysical Journal. To reach their conclusion, Perets and Kouwenhoven simulated young star clusters containing free-floating planets. They found that if the number of rogue planets equaled the number of stars, then 3 to 6 percent of the stars would grab a planet over time. The more massive a star, the more likely it is to snag a planet drifting by.”
The article on spaceref.com goes on to say: “Stars trade planets just like baseball teams trade players,” said Hagai Perets of the Harvard-Smithsonian Center for Astrophysics.” The article on spaceref.com says further: “A captured planet tends to end up hundreds or thousands of times farther from its star than Earth is from the Sun. It’s also likely to have a orbit that’s tilted relative to any native planets, and may even revolve around its star backward.” Retrograde orbits are specifically mentioned here.
So, while I would agree that a hypervelocity rouge planet would be difficult to capture, all rouge or orphan planets are not traveling at hypervelocity.

The likely hood of a planets capture would be heightened by a solar nebula acting as a break during the early formation of a planetary system, angular momentum must be shed to enter into a stable orbit, gas clouds would allow this to happen.

Andy makes a good point, and I agree that for planets such as HAT-P-7b “sequential Kozai migration” is definitely a possible explanation. However, that mechanism is much more complicated than the capture of an orphan planet that is already in a retrograde orbit. While the article I cited does indicate that captured planets are “typically” in much wider orbits than HAT-P-7b, it does not rule out the possibility. Planetary migration is still the most likely explanation for hot Jupiters with orbits of only a few days. If an orphan planet were captured in a retrograde orbit during the late phase of planetary system formation, then the angular-momentum exchange between the captured planet and the native planets and planetesimals could have moved it close enough to be affected by Type II migration that would have moved it even closer to the central star. Capture of a retrograde orphan planet in an earlier phase of planetary system formation would only increase the likelihood of inward migration.
I should point out that while retrograde planetary capture and planetary migration may account for a planet similar to HAT-P-7b, I lack the expertise rigorously to compare the “simplicity” of that scenario with “sequential Kozai migration” with any kind of mathematical precision. But just from an intuitive perspective it is simpler to start out with retrograde motion and migrate inward than completely to reverse a planet’s orbit, especially when we are talking about a Jupiter sized planet in an orbit as tight as HAT-P-7b.

Usually, the equation of orbital velocity of a planet of our solar system is found out by assuming that the centripetal force, required for the planetary motion, is provided by the gravitational force. This leads us to the famous equation of orbital velocity that is V^2 = GM / R. This raises the question how to get the equation of the velocity of a “retrograde” exo-planet. This is because, I think, we can not use the said assumption for forward as well as retrograde motion. Even students will question the said assumption – at least latently. Read my letter in Physics Education, UK, January 2012, p. 132. Or feel free to contact me using dvsathe@gmail.com

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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